Aromatic aldehydes have aldehyde groups having high reactivity, and so have a wide range of usages. Especially, terephthalaldehyde having two aldehyde groups at para position has received an attention as raw material for medicinal products, agrochemicals, pigments, liquid crystal polymers, or plastic having heat resistance.
Conventional methods for preparation of terephthalaldehyde known to the art are hydrolysis method of intermediate prepared by chlorination of p-xylene, and hydrogenation method of dimethyl terephthalate. However, these conventional methods are not suitable for economical mass production since their processes are complicated and should be carried out under high pressure and environment-unfriendly conditions.
In order to solve these problems, there has been a continuous study for mass production of terephthalaldehyde by gas-phase-oxidizing p-xylene with molecular oxygen.
For example, Japanese Patent Publication No. 47-002086 discloses a complex oxide catalyst having the ratio range of W:Mo of from 1:1 to 20:1. And, Japanese Patent Publication No. 48-097830 discloses a catalyst comprising V and Rb or Cs. U.S. Pat. No. 3,845,137 discloses a catalyst consisting of two components, W and Mo, and one or more components selected from the group consisting of Ca, Ba, Ti, Zr, Hf, Tl, Nb, Zn, and Sn. Also, U.S. Pat. No. 4,017,547 discloses a catalyst consisting of Mo oxide, W oxide or silicotungstic acid and Bi oxide. However, the industrial practical uses of these catalysts have been limited due to the low selectivity and yield of terephthalaldehyde.
Also, U.S. Pat. No. 5,324,702 discloses a catalyst comprising a first component selected from the group consisting of Fe, Zn, Zr, Nb, In, Sn, Sb, Ce and Bi, and a second component selected from the group consisting of V, Mo and W, wherein the first and second components are distributed on a deboronized borosilicate crystal molecular sieve by chemical vapor deposition (CVD). This catalyst shows relatively higher conversion rate to p-xylene, and relatively higher yield of terephthalaldehyde, than conventional catalysts. However, the catalyst also has a limitation in increasing the selectivity of various by-products, and so was difficult to separate and purify it.
Moreover, U.S. Pat. No. 6,458,737 discloses a catalyst comprising a major component of W, and one or more components selected from the group consisting of Sb, Fe, Co, Ni, Mn, Re, Cr, V, Nb, Ti, Zr, Zn, Cd, Y, La, Ce, B, Al, Tl, Sn, Mg, Ca, Sr, Ba, Li, Na, K, Rb and Cs. The catalyst can provide a high yield of terephthalaldehyde enough to have industrial applicability. However, the catalyst also has a limitation in the separation and purification since the selectivity of terephthalaldehyde is not high compared with high conversion rate of p-xylene. Further, the catalyst has problems in heat stability and life since it comprises Sb component which is sublimated and lost at high temperature.
In short, in case of using these conventional catalysts, the yield of terephthalaldehyde is low. Or, the selectivity is low even though the yield is high. Thus, the separation and purification are difficult. Also, it is difficult to prepare the catalysts to have homogeneous composition and performance since they use complex oxide having multiple components. Further, the catalysts comprise components having low heat stability, and so have short life span, and thus their industrial practical uses have been limited.
On the other hand, Korean Patent Application No. 10-2004-0089376 filed by the present inventor disclosed a single-component catalyst comprising tungsten oxide, and fire-resistant inorganic carrier as optional component. The catalyst has advantages that it is easy to make it homogenous, and it has higher selectivity and yield of terephthalaldehyde than conventional complex oxide having multiple components. However, the catalysts having higher selectivity or terephthalaldehyde are still required